Traditionally, health monitor devices were only applicable in fixed hospital/medical settings, using expensive, high tech machinery relying on trained medical personal. Recently however, portable health monitoring devices have become a growing trend in modern healthcare to provide continuous physiological information to the wearer and/or to guide healthcare practitioners in their medical decisions. Continuous access to physiological parameters can be widely beneficial in a scientific and clinical research setting, as well as in the sport and fitness sector to motivate and guide users to achieve and maintain personal health, wellness and fitness goals.
Photoplethysmography (PPG) is a well-known optical sensing technique used to measure the hemodynamic properties in organisms. Transcutaneous PPG technology essentially entails the illumination of the skin of an organism and the subsequent photo-detection of reflected or transmitted light through a sample in order to measure small changes in light intensity. These small changes in light intensity are due to shifts in the concentration of several subcutaneous chromophores (molecules that absorb light). These concentration changes occur due to changes in subcutaneous blood volume due to the level of tissue perfusion as a result of (i) expansion and contraction of capillary vessels during the cardiac cycle and (ii) subcutaneous fluid movement due to motion of the organism. Several transformations of the PPG signal have been proposed and implemented to obtain physiological parameters from the PPG signal, and several algorithms and embodiments have been proposed to yield accurate, motion resistant physiological parameters, such as heart rate and oxygen saturation, from wearable PPG sensors.
Progress has been made to reduce the cost of wearable PPG-based sensors that are resistant to motion artifacts, but most of the PPG solutions currently employed use a combination of optical and accelerometer based solutions to accurately determine heart rate. US Patent Application Publication 20140213863 to Texas Instruments Inc. describes a PPG-based heart rate monitor is that uses an optical motion sensor to remove artifacts. A reference signal for motion compensation is generated either by a incorporating a second light emitting diode (LED) of a different wavelength, alternatively, by either lowering the driving current of the primary LED or by changing the wavelength of the primary LED, which is subtracted from the desired heart rate signal to correct for motion artifacts. In U.S. Pat. No. 8,483,788 to Covidien Lp. a motion compensated pulse oximeter is described that includes an accelerometer to measure the changes induced by motion between the LED and the photodetector. Furthermore, US 20140058217 described a pulse PPG device including motion artifact mitigation methods. Here, a first circuit comprises a PPG-based sensor, while a second circuit comprised of an impedance-based sensor for surface motion artifacts. Currently, PPG technologies are still inherently susceptible to motion artifacts, therefore, they are limited to measure heart rate and oxygen saturation as there is no PPG based device that resolves this problem satisfactory during severe bouts of physical activity.
Furthermore, currently no device, applied to a single point (defined as a surface not larger than a circle with 1.5 inch radius) on the body, can accurately gauge the velocity of a heart beat pulse propagated through the arterial circulation. This phenomenon is known as pulse wave velocity and there exist several optical based solutions where optical sensing modules are placed on different sites of the organism. For example, U.S. Pat. No. 7,674,231 B2 describes a wearable pulse wave velocity blood pressure sensor to perform circulatory measurements on an extremity of a subject. Here, a first PPG signal is obtained from a first position on the subject (such as the wrist) and a second PPG signal at a second position on the extremity (such as a digit). Alternatively, combination approaches exist where an optical sensing module and ECG are employed. For example, in U.S. Pat. No. 6,331,162 B2 claims to analyze blood flow by recording PPG-based waveforms at two locations along the descending thoracic aorta, while simultaneously recording the ECG waveform of the subject in order to determine pulse wave velocity.